Gel-type electrolyte

Both the anode and the cathode are composed of a coating of the electrochemically active material onto a current collector (copper or aluminum). Another key component of the battery is the separator that physically separates the two electrodes and prevents contact between them. In the case of a liquid technology battery, a polyolefin separator is typically used and a liquid electrolyte is used to transport the Li ions from one side of the porous separator to the other. In the case of a polymer Li ion battery, a polymer, such as PVDF, is used to form a porous structure, which is then swollen with a Li" " conducting liquid electro-lyte. " This results in a gel-type electrolyte, which plays the dual role of electrolyte and separator, with no free liquid present. 

A variety of dimensionally stable solid electrolytes consisting of a mixture of organic plasticizers such as EC, PC etc., along with structurally stable polymers such as poly( acrylonitrile) (PAN) or poly( vinyl sulfone) (PVS), or polyvinyl pyrrolidine (PVP) or polyvinyl chloride (PVC) and several lithium salts have been tested and found to have excellent ionic conductivities at ambient temperatures [155-156]. In these gel type electrolytes the primary role of the polymers PAN, PVS, PVP or PVC is to immobilize the lithium salt solvates of the organic plasticizer liquids. However, with polymers such as PAN a coordination interaction with Li+ is also quite likely. 

An alternative to lithium batteries with liquid electrol5des are those with solid polymer electrolytes. Solid polymer electrodes are generally gel type electrolytes which trap solvent and salt in pores of the polymer to provide a medium for ionic conduction. Typical polymer electrolytes are shown in Table 15.8. 

Feullade and Perche [67] originally introduced gel-type electrolytes. However, interest in these materials has increased recently with the development and the characterisation of many new types of membranes having different properties [59, 68-74], Some examples of these membranes with their composition and their electrochemical properties are listed in Table 7.2 [72]. 

Clearly, these gel-type electrolytes have quite promising properties in terms of conductivity, approaching that of liquid solutions. This can be seen in Figure 7.8, which shows the Arrhenius plots of some selected examples, and Figure 7.5 which compares the conductivity of gels with that of PEO-based membranes. 

Figure 7.8 Conductivity Arrhenius plots of various gel-type electrolytes. The plot of a typical liquid solution is also reported for comparison purposes.

One may then conclude that, the gel-type electrolytes, and the PAN-based ones in particular, have electrochemical properties that in principle make them suitable for application in versatile, high-energy lithium batteries. In practice, their use may be limited by the reactivity towards the lithium electrodes induced by the high content of the liquid component. Indeed, severe passivation phenomenon occurs when the lithium metal electrode is kept in contact with the gel electrolytes [60, 69]. This confirms the general rule that if from one side the wet-like configuration is essential to confer high conductivity to a given polymer electrolyte, from the other it unavoidably affects its interfacial stability with the lithium metal electrode. 

Once the compatibility of the gel-type electrolyte with both anode and cathode materials is ascertained, one can proceed with the combination of the two for the fabrication of polymer-based lithium-ion battery prototypes. A few examples of these prototypes have been reported at the laboratory level scale. One is provided by a battery of the type C/ LiC104-EC-PC-PAN/LiCryMn2.y04. 

Other recent developments include the incorporation of a fire retardant, which retards the combustion of the solvent, and a new additive to improve the wetting of the separator. It is difficult to use these additives in the gel-type electrolytes employed in lithium-ion polymer cells. This may be one reason for the lower market share experienced by lithium-ion polymer cells. 

Generally the polymer electrolyte of the polymer battery is classified into two kinds of the electrolyte One is a dry-type electrolyte composed of a polymer matrix and the electrolyte salt the other is a gel-type electrolyte in which polar solvent is added as a plasticizer to appropriate polymer matrix. In addition, the gel-type electrolyte is classified into thermoplastic gel and cross-linked gel. The research flow of SPE is shown in Fig. 21.1. 

Semisealed lead calcium. A gel-type electrolyte is used that does not require the addition of water. There is no outgassing or corrosion. This type of battery is used when the devices are integral to small UPS units, or when the batteries must be placed in occupied areas. The fife span of a semisealed lead calcium battery, under ideal conditions, is about 5 years. 

A gel-type electrolyte, comprising polymers such as polyacrylonitrile or poly vinylidene di fluoride-hexa fluoro propiate (PVdF-HFP) EC-PC as plasticizer and LiC104. ... 

Elucidating the role of various plasticisers in gel-type electrolytes. 

The polymer-lithium salt system seems to be the most widely studied polymer electrolyte, due to its potential application in lithium high density batteries and ECD. Such electrolytes should be completely anhydrous and therefore, must be prepared and kept under moisture-free conditions. While gel-type electrolytes based on the PEO-KOH " and PVA-KOFI-H2O alkaline SPE systems have been reported, such alkaline electrolytes are interesting from the point of view of their potential application in all-solid alkaline rechargeable batteries and ECD. 

Gel-type electrolytes (GPEs) are hybrid systems that may be described as formed by a liquid component contained within a polymer network. 

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